Vinblastine is a clinical drug used in frontline combination therapies for treatment of cancer. It acts by inhibition of mitosis through binding tubulin and disrupting microtubule formation. Because of advances in its total synthesis, we report previously inaccessible and unusual modifications to vinblastine that improve potency a remarkable 100-fold. These ultrapotent vinblastines display much higher tubulin binding affinities and likely further disrupt the tubulin head-to-tail dimer–dimer interaction by strategic placement of an added rigid, extended group along the adjacent continuing protein–protein interface. Significantly, the ultrapotent vinblastines are accessible by chemical synthesis in three steps from commercially available materials (catharanthine, $16/g; vindoline, $36/g) based on newly introduced synthetic methodology and are inaccessible by natural product derivatization, late-stage functionalization, or biosynthetic methods.